2. Electrical Activity of the Heart Flashcards
What is the sarcolemma?
Membrane which surrounds the heart cells which is invaginated
What is the sarcoplasmic retriculum?
Big calcium store which is needed to trigger excitation
What type of junctions are found between cardiac cells and what do they allow?
Gap junctions; allow electrical connection and passage of signalling molecules from one cell to the next (depolarisation and APs pass from cell to cell)
What ensures a physical connection between adjacent cardiac cells?
desmosomes
What is the structure involving desmosomes and gap junctions called?
Intercalated discs
What is the difference between AP in skeletal and cardiac muscles?
AP in cardiac muscles is much longer and larger
What is needed in cardiac cells to create longer APs?
Bigger influx of Ca from outside into the cell
What contraction is created from ca entry into the cardiac cell?
sub-maximal contraction
Is there tetanus in cardiac muscle?
No tetanus; no prolonger contraction of muscle by prolonged repeated stimuli (tetanus exists in skeletal muscle)
What is the refractory period like in cardiac muscle cells?
Long refractory period; cardiac muscle has to relax before it can contract again
What are contractions in cardiac cells like?
they are GRADED contractions (whereas skeletal muscle ones are all or none)
What is the force generated by cardiac muscle proportional to?
Proportional to number of cross bridges that are active which is determined by how much Ca is bound to troponin
What is the contraction of cardiac muscle dependent on? (2)
- cytosolic Ca concentration
2. sarcomere length at the beginning of contraction (initial length of muscle fibre)
What is the main difference in action potentials between skeletal and cardiac cells?
In cardiac cells;
- rapid depolarization occurs because of Na entry
-steep repolarization occurs because of K leaving
BUT! In cardiac cells the main difference is that there is the lengthening of the AP due to Ca entry
Cardiac muscle cells act as one functioning cell- what is this called?
functional syncytium
How are cardiac cells electrically connected?
via gap junctions
How are cardiac cells physically connected?
via desmosomes
What is the AP of a cardiac muscle in terms of msec compared to skeletal muscle?
Cardiac muscle AP= 250msec (much longer AP)
Skeletal muscle AP= 2msec
What is the refractory period of cardiac muscle?
Long so cannot exhibit tetanic contraction
What regulates contraction force on a molecular level?
Ca entry from outside cell (can be used to vary strength of contraction)
What do pacemaker cells have that other cardiac cells don’t in terms of potentials?
they have an unstable resting membrane potential
Why is prevention of having tetanus important in cardiac cells? Why is it NOT needed?
Because cardiac muscles must relax between contractions so ventricles can fill with blood
What is tetanus?
Sustained contraction (when a series of APs occur in rapid successions)
Why can’t tetanus never occur in cardiac cells?
Because longer AP means refractory period and contraction end almost simultaneously (by the time the second AP takes place, the myocardial cell has almost completely relaxed)
Can summation occur in cardiac cells?
NO
What is the RMP of non-pacemaker cells? What are they described as?
RMP= -90mVP
Very stable cells
What is the RMP in pacemaker cells? What are they described as?
RMP= -60mVP
Unstable
Why is the potential for pacemaker cells called “pacemaker potential” rather than RMP?
Because it never rests at a constant value
What is the main difference between depolarisation between non-pacemaker and pacemaker cells?
In pacemaker cells depolarisation is due to additional Ca channels opening while in non-pacemaker cells this is due to Na channels opening
What creates an RMP in normal non-pacemaker cells?
leaky K channels
Describe the 4 steps in non-pacemaker APs by referring to its basic electrophysiology.
- Resting Membrane Potential
- Initial depolarisation
- Plateau
- Repolarisation
What occurs in step 1; resting membrane potential?
High resting PK+ (leaky K channels)
What occurs in step 2; initial depolarisation?
- increase in PNa (influx into the cell)
What occurs in step 3; plateau?
-increase in PCa (L type; long type)
- decrease in PK
(flattened AP)
What occurs in step 4; repolarisation?
- decrease in PCa
- increase in PK
(reverse of Plateau)
What step in electropysiology is specific to cardiac cells?
Plateau stage (which keeps Ca channels open for longer to keep the AP going)
What is the AP in PACEMAKER cells due to?
Increase in PCa (l type); rather than PNa increase like in non-pacemaker cells
If pacemaker cells don’t have an RMP, what do they have instead?
Pacemaker potential (or pre-potential)
What 3 features are specific to pacemaker APs?
- gradual decrease in PK
- early increase in PNa (Pf)
- late increase in PCa (T-type)
What do pacemaker create?
Autorhythmicity
Early increase in PNa (pf) is due to Na channels being open from which stage?
These channels are open from previous repolarisation from a previous AP not due to depolarisation
What are 2 common drugs used that affect forces of contraction?
- Ca channel blockers
2. Cardiac glycosides
How do Ca channels blockers affect force of contraction
decrease force of contraction; less Ca released, less bridges formed and weaker contraction
How do cardiac glycosides affect force of contraction?
increase force of contraction; more bridges formed and stronger contraction
How does temperature affect heart rate?
Increases heart rate ~10 beats/ min/centigrade
How does hyperkalemia (high plasma K) affect heart rate?
causes fibrillation and heart block: decreases equilibirum potential and cell is depolarised, APs are uncoordinated
How do hypokalemia (low plasma K) affect heart rate?
causes fibrillation and heart block (same as hypokalemia)
How does hypercalcemia (high plasma Ca) affect heart rate?
Increases heart rate and force of contraction
How does hypocalcaemia (low plasma Ca) affect heart rate?
decreases heart rate and force of contraction
What are modulators which affects electrical activity of the heart? (7)
- sympathetic and parasympathetic nervous systems
- drugs (Ca channel blockers and cardiac glycosides)
- temperature
- hyperkalemia
- hypokalemia
- hypercalcemia
- hypocalcemia
Where are fastest pacemaker cells found?
At sinoatrial node (controls heart rate); pacemaker ~0.5m/sec
- generates cardiac impulse
What is annulus fibrosus?
- Insulator between atria and ventricles, it’s a right fibrous ring surrounding the heart
- strongest part of fibrous cardiac skeleton
Is annulus fibrosus conducting?
it’s NON-CONDUCTING
What is the atrioventricular node (AV node)?
- slows down impulses and regulates them
- found inferior to SA node
- secondary pacemaker and allows atria to work
- delays APs to ventricle
- relays and intensifies cardiac impulse
What is the AV node regarded as?
- the “delay box”
- ~0.05m/sec
Where are Bundle of His found?
fibres around ventricles for equal ventricular contraction
What does Bundle of His divide into?
divides into Purkinje fibres
What do Purkinje fibres do?
- rapid conduction system
- ~5m/sec
- makes sure depolarisation occurs at the same time
Where is SA node located?
superior lateral wall opening of superior vena cava
Where is the AV node located?
posterior septal wall of right atrium
Is SA or AV node shorter?
SA node is longer and AV node is shorter
To what does SA node transmit impulse to?
directly to two atria
To what does AV node transmit impulse to?
to two ventricles through AV bundle (influenced by SA node)
Can an AP in a single myocyte evoke a very small extracellular (transmembrane) electrical potential?
Yes
How can large extracellular electrical waves be created?
Lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time can summate to create these
What can be used to record large extracellular waves?
They can be recorded at the periphery as the electrocardiogram
What does P wave correspond to?
atrial depolarisation (systole)
What does QRS complex correspond to?
ventricular depolarisation (systole)
What does T wave correspond to?
ventricular repolarisation (depolarisation)
What does pre-potential in pacemakers involve?
- decrease in K
- increase in Na (NaF)
- increase in Ca (T type) permeability
What does AP in pacemakers involve?
- increase in Ca (l type) permability
What is 1st degree heart block?
Heart’s electrical signals/ conduction are slowed down as they move from atria to ventricles
How does 1st degree heart block look like on an ECG?
longer and flatter waves between P and R in an ECG
What is 2nd degree heart block?
- Electrical signals/ conduction are slowed down between atria and ventricles to a large degree and some signals don’t reach ventricles
- less contraction of ventricle
How does 2nd degree heart block look like on an ECG?
On an ECG, the pattern of QRS complex wave doesn’t follow each P wave as it normally would
What is 3rd degree heart block?
None of the electrical signals reach the ventricles
- is also called complete heart block or complete AV block
How does 3rd degree heart block look like on an ECG?
P wave occurs at a faster rate and isn’t coordinated with QRS waves (normal pattern is disrupted)
What are some examples of arrhythmias?
- atrial fibrillation
- atrial flutter
- ventricular fibrillation
What is atrial fibrillation?
fast and irregular contraction of the atria
What is atrial flutter?
- similar to atrial fibrillation
- main difference is that electrical signals are spread through the atria in a fast and regular pattern (rather than irregular like in fibrillations)
What is ventricular fibrillation?
disorganised electrical signals make ventricles quiver and send irregular signals
